1. Field of Invention
The invention relates to a brightness adjustment circuit, and in particular, to a brightness adjustment circuit for an electroluminescent display.
2. Related Art
In the industry of flat-panel displays, organic light-emitting diode (OLED) displays have received a lot of attention in recent years due to its self-luminescence, high brightness, super-wide viewing angle, high response speed, low working voltage, and light weight. However, the electroluminescent displays only have a limited market to date. Various display manufacturers have been trying to find an optimal manufacturing method for improving the yield product properties in order to make them more popular.
The OLED display is a self-luminescent flat-panel display. Its light source is produced by converting from a current flowing through the OLED pixels. By adjusting the current, we can determine the maximum brightness of the panel. Different gray levels are defined by the coupled thin film transistors (TFTs). The display thus achieves full colors and multiple color levels.
The working voltage (Vdd or Vss) of each pixel unit of the OLED is supplied by an external source. The power consumption required by all the light-emitting units in the OLED panel is provided by this single power supply system. In the circuit design, all the working voltage input terminals Vdd of all the pixels are connected in parallel. Likewise, the terminals Vss are all coupled in parallel, before individually guiding them to the outermost edge of the panel and to the external power supply system via a flexible print circuit (FPC) or wires.
The external power supply is designed as a stable voltage source system. That is, it is designed to have a fixed voltage across its output terminals. This ensures that the output voltage does not vary due to an unstable input voltage or noise interference. The ordinary stable voltage source system uses a voltage feedback control means. FIG. 1 is a schematic view of a conventional feedback stable voltage power supply unit. The power supply unit 100 has a power supply 102 and a feedback circuit. The power supply 102 has a voltage output terminal Vout and a feedback terminal 108. The voltage output terminal Vout provides the work voltage (Vdd or Vss) required by the pixel units (not shown). The feedback circuit includes two resistors 104, 106 and is connected to the pixel units in parallel. One end of the resistor 104 is electrically coupled to the voltage output terminal Vout, and one end of the resistor 106 is electrically coupled to a reference voltage source. The feedback terminal 108 is electrically coupled between the resistor 104 and the resistor 106. The resistors 104, 106 have the effect of dividing the potential difference. The purpose is to produce a reference voltage (generally 1.25V) set for the system and feed it to the feedback terminal 108, monitoring the stability of the output voltage. When the reference voltage increases, the feedback terminal 108 sends out a signal to reduce the output voltage of the voltage output terminal Vout, bringing the reference voltage back to 1.25V. On the other hand, when the reference voltage decreases, the feedback terminal 108 sends out a signal to increase the output of the output voltage terminal Vout. In either case, the reference voltage is brought back to 1.25V.
However, the prior art shown in FIG. 1 only controls the stability in the brightness of the pixel units instead of adjusting the brightness. For solving problems in lifetime and residual images, some operating mode has to be provided to elongate the lifetime of the material and panel. For example, it is better not to have images with fixed positions and not to use high-brightness images for a long time. High brightness and continuous ON of the OLED are the primary reasons for shortening the panel lifetime. In view of the two consequences mentioned above, one can adopt the scheme of intermittent light-ups, e.g. 10 or 20 seconds. Afterwards, the brightness of the screen is reduced to one half or ⅓. This method can increase the panel lifetime without sacrificing its practical uses.
Therefore, a circuit that can adjust the brightness of the pixel units has been proposed in the prior art. Suppose the pixel units are required to switch among four different brightness modes, the feedback stable power supply unit has to provide a switch for four voltage outputs. A conventional method is to provide four different feedback resistor sets along with a channel switch for producing different output voltages. As shown in FIG. 2, the conventional power supply unit 200 that can adjust the brightness of the pixel units has a power supply 202 and a feedback circuit. The power supply 202 has a voltage output terminal Vout and a feedback terminal 204. The voltage output terminal Vout provides the working voltage (Vdd or Vss) required by the pixel units (not shown). In particular, Vdd is used on the pixel units driven by P-type metal oxide semiconductor (MOS) transistors, whereas Vss is used on the pixel units driven by N-type MOS transistors. The feedback circuit includes four sets of serial resistors connected in parallel. Each set has two resistors in series and is connected with the pixel unit in parallel. The four sets of serial resistors include resistors 208, 210, resistors 214, 216, resistors 220, 222, and resistors 226, 228. One end of each of the resistors 208, 214, 220, 226 is electrically coupled to the voltage output terminal Vout, and one end of each of the resistors 210, 216, 222, 228 is electrically coupled to a reference voltage. The four sets of serial resistors 208 and 210, 214 and 216, 220 and 222, and 226 and 228 have connection points 212, 218, 224, and 230, respectively. The switch 206 and the feedback terminal 204 are electrically connected. The switch 206 can be electrically connected to the connection points 212, 218, 224, and 230. The four sets of serial resistors 208 and 210, 214 and 216, 220 and 222, and 226 and 228 can be designed to have different resistance values, so that the switch 206 provides different feedback voltages on the feedback terminal 204 when it is electrically connected to the connection points 212, 218, 224, and 230. The output of Vout of the power supply 202 is adjusted to change the brightness of the pixel units. However, the conventional design of the brightness adjustment circuit occupies a substantial area in the entire circuit. Therefore, how to effectively reduce its area is an important issue in the field.